1. Field of the Invention
The present invention relates to sheath flow techniques, and more particularly to a method of positioning a laminar flow of a sample fluid on a surface within a flow cell by guiding the sample fluid flow between two laminar flows of guiding fluids. The invention also relates to an analytical system, a computer program product and a computer system for performing the method.
2. Description of the Related Art
WO 99/36766 discloses methods for selectively contacting defined lanes or strips of a surface within a flow cell with a fluid capable of interacting therewith, such as a sensitizing fluid or a sample fluid, using a laminar flow technique usually referred to as hydrodynamic addressing. In one embodiment, the laminar flow of the sensitizing (or sample) fluid is guided between two buffer flows and may be positioned as desired over the surface by adjusting the relative flow rates of the buffer flows. This may be accomplished in a flow cell of the type having three inlets and one outlet (a so-called Ψ- or psi-cell), the interacting fluid being introduced through the central inlet and the guiding buffer fluids through the other two inlets.
Methods for detecting interactions of a fluid with the flow cell surface in WO 99/36766 include surface plasmon resonance (SPR) spectroscopy which measures refractive index changes at the surface caused by the interactions. Since the SPR detection response is influenced by the refractive index of the fluid medium in the vicinity of the surface, SPR detection may also be used to detect the position of a sensitizing or sample fluid flow on the surface when calibrating the flow cell with respect to interface position and flow rates. This may be done by replacing the interacting fluid by a fluid having approximately the same viscosity but which does not interact with the surface and which has a different refractive index than the buffer. Thereby each interface between the sample fluid and the two buffer flows can be detected. When the flow cell has been calibrated, each lateral position of the interacting fluid flow corresponds to a defined setting of the relative flow rates of the buffer flows.
WO 00/56444 discloses a similar use of a Ψ-cell as the above-mentioned WO 99/36766. In one embodiment, parallel multiple sample streams, separated by guiding streams, are passed through a flow channel. To position the sample streams, two main guidance streams are first introduced, and their flow rates are adjusted to produce an interface at a desired lateral position. The flows of the sample streams and their guiding streams are then introduced between the two main guidance streams.
It is an object of the present invention to provide an improved method of positioning the central fluid flow in a flow cell of the Ψ-cell type.